With the active composting season beginning in many climates, it may be worthwhile going over what you should not put in the compost bin.
Obviously, you would never put glass, metal, non-biodegradable plastic or other synthetic materials in a composter: they simply don’t break down! But there are also some organic products is it best to avoid, at least when it comes to home composting.
This is the case of animal products (fat, meat, pet feces (including cat litter), disposable diapers, etc.) because of the undesirable microbes that can develop. Of course, compost that heats up thoroughly should be able to destroy even the most harmful microbes, but home composters don’t have the reputation of generating much heat. Also, the smell of the meat tends to attract vermin. For these reasons, it is best not to compost animal products.
Also avoid composting seed-bearing weeds because, again, the seeds might not be destroyed by heat and then the compost you produce could spread weeds when you use it.
The same goes for the roots of weeds with spreading rhizomes (horsetail, goutweed, Japanese knotweed, quackgrass, etc.) unless you have killed the rhizomes beforehand (you could kill them soaking them in water until they start to rot). Although there is a good chance that they will decompose in many compost bins, why take unnecessary risks?
Beware too of organic products that are very slow to decompose. Branches, logs and lumber take ages to decompose thoroughly, for example, unless you pass them through a tree shredder. And corn cobs and fruit stones are just as slow. It may also be wise to shred oak leaves and conifer needles to hasten their decomposition.
Many people put eggshells and oyster shells in the compost, which is not bad in itself, but do at least reduce them into a fine powder first, or they make take years to decompose. Of course, even if they don’t decompose much, at least you’ll be recycling them, but it’s still best to grind them up thoroughly if you expect them to contribute much to the quality of your compost.
Finally, animal bones left over from the table should not be composted for two reasons: risk of contamination and their extremely slow rate of decomposition.